Contemporary automotive air conditioning systems typically use parallel flow condensers that are fabricated with extruded tubing. This tubing, which is referred to as micro-multiport (MMP) tubing, is made from 1XXX or 3XXX Al alloys. The tubing is a flat body with a row of side-by-side passageways, which are separated by upright webs. Processing of this tubing involves extrusion, a straightening and cutting operation, assembly and furnace brazing of the condenser. Brazing is generally done at 600.degree.-605.degree. C. (about 94% of the melting temperature of pure Al). The tube straightening operation imposes a small amount of cold work, in the critical range, which causes extremely coarse grains to grow during the brazing process.
Material handling involves winding the tube on coils and transferring these coils to a straightening and cutting operation. It is during this operation that the final width, thickness and length dimensions of the cut pieces are achieved. The cut pieces are then assembled into a condenser core with fin stock and headers that are clad with a brazing alloy. This assembly is brazed at 600 to 605.degree. C.
The production of automotive condensers from aluminum MMP tubing involves an interaction of process conditions that can result in undesirable material properties. The combination of a small amount of cold work and the high brazing temperature that must be imposed on the tube cause extremely large grains to form, and this has a significant effect on mechanical properties.
Small amounts of cold work are imposed on the tube during straightening/sizing and material handling. This small amount of deformation can lead to a phenomenon in which very large grains in the aluminum are formed during the brazing process. If a critical amount of cold work is imposed on the tube prior to brazing, then extremely large grains will form after recrystallization. The critical amount of cold work is defined as the amount of strain just necessary to initiate recrystallization. Since few nuclei are formed in the metal, the growth of relatively few recrystallized grains is allowed to proceed with minimum resistance. Conversely, as the amount of cold work increases, more nuclei are produced and the recrystallized grain size decreases.
This invention improves the grain size and the metallurgical strength of the tube by cold working the webs in the tubes and controlling the grain size. The webs in the tube body between each pair of said passages are substantially hour glass shape, namely, an upright wall with a reduced thickness section substantially midway between the top and bottom ends of the wall. In response to successive cold working of the body, the webs are changed in shape from the hour glass shape to a more uniform thickness shape. Stated otherwise, in this invention, the sides of the webs are tapered at an angle such that when there is a 5% change in material thickness, the strain is concentrated at the center of the web and results in at least 15% cold work. At 15% cold work or more the amount of grain growth will be controlled.
Accordingly, this invention provides an improved process for enhancing the metallurgical strength of a multiport tube for use in a condenser or an evaporator. The invention provides a multiport tube which includes webs between the ports that are configured such that when there is a five percent change in material thickness, the strain from cold working of the tube is concentrated at the center of the webs to improve the strength of the tubing and maintain the desirable small grain growth in the metal tube.
Further objects, features and advantages will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.